Vol. 70.] THROUGH THE ANDES OF PERU AXD BOLIVIA. 39 



American forms, states that the chief distinction lies in the more 

 marked ribbing of the former and in the presence of a narrow groove 

 extending from the apex of the brachial valve. As most of my 

 specimens are either in a poor state of preservation, or in the form 

 of casts, I am unable to express an opinion on this point, although 

 obsolescence of the lateral ribs is to be observed in several instances. 



Leptocozlia acutipltcata (Conrad). (PL VIII, fig. 4.) 



The numerous examples of this species agree closely with the 

 form described by Dr. Ivor Thomas. 1 



Leptoccelia flabellites (Conrad), as interpreted by Dr. Ivor 

 Thomas. 2 



Scaphiocozlia b OLIVIER sis, Whitfield. 



The genotype of this interesting shell was first described by 

 Whitfield 3 from near Sucre in Bolivia, and appears to be charac- 

 teristic of the lowest beds of the ' Icla-Schiefer,' which probably 

 correspond to the Lower Helderberg Group. 



The specimen in my collection was obtained by one of the 

 engineers from a railway- cutting between Coniri and Viacha, but 

 the exact locality was not recorded. 4 



(e) The Alluvial Deposits of the Eastern Altaplanicie. 



The folded Palaeozoic rocks which form the basement of the 

 Altaplanicie disappear from view beyond the town of Viacha, and 

 are not seen again until the eastern range, known as the ' Cordillera 

 Real,' is reached, where they once more crop out with steep dips 

 along the flanks of this lofty chain. The intervening country is 

 buried beneath a thick mantle of alluvium, which forms a wide 

 desert-plain between Viacha and the Alto de La Paz, attaining 

 a mean altitude of nearly 13,000 feet. On the north lies 

 Lake Titicaca and some distance away to the south Lake Poopoo, 

 the two being connected by the Desaguadero River. 



One of the chief points to be taken into consideration in dis- 

 cussing the origin of these alluvial deposits of the Altaplanicie 

 is the presence at a considerably lower altitude of deep gorges, such 

 as that of the La Paz River, cut right through the Cordillera. 

 These, from their great depth and the hard nature of the rocks 

 which they traverse, would appear to have been in existence at 

 the time of maximum extension of the lake-system, and thus must 

 have formed a natural outlet to the eastward. 



The fact that there is no evidence showing that at any time the 

 waters were diverted in this direction is probably to be explained 

 by the supposed existence of a ridge of Palaeozoic rocks running 



1 Zeitschr. Deutsch. Geol. GeseUsch. vol. lvii (1905) p. 263 & pi. xiv, 

 figs. 36 a-c, 37 



2 Ibid. pi. xiv, figs. 35-35 c. 



3 Trans. Am. Inst. Min. Eng. vol. xix (1890-91) pp. 105-107 & figs. 1-4. 



4 See also E. Knod, Neues Jahrb. Beilage-Band xxv (1908) p. 555 & pi. xxix. 

 figs. 1-5. 



